Comparison of pulsed three-dimensional CEST acquisition schemes at 7 tesla: steady state versus pseudosteady state

Abstract

PurposeTo compare two pulsed, volumetric chemical exchange saturation transfer (CEST) acquisition schemes: steady state (SS) and pseudosteady state (PS) for the same brain coverage, spatial/spectral resolution and scan time.MethodsBoth schemes were optimized for maximum sensitivity to amide proton transfer (APT) and nuclear Overhauser enhancement (NOE) effects through Bloch‐McConnell simulations, and compared in terms of sensitivity to APT and NOE effects, and to transmit field inhomogeneity. Five consented healthy volunteers were scanned on a 7 Tesla Philips MR‐system using the optimized protocols at three nominal B1 amplitudes: 1 μT, 2 μT, and 3 μT.ResultsRegion of interest based analysis revealed that PS is more sensitive (P < 0.05) to APT and NOE effects compared with SS at low B1 amplitudes (0.7–1.0 μT). Also, both sequences have similar dependence on the transmit field inhomogeneity. For the optimum CEST presaturation parameters (1 μT and 2 μT for APT and NOE, respectively), NOE is less sensitive to the inhomogeneity effects (15% signal to noise ratio [SNR] change for a B1 dropout of 40%) compared with APT (35% SNR change for a B1 dropout of 40%).ConclusionFor the same brain coverage, spatial/spectral resolution and scan time, at low power levels PS is more sensitive to the slow chemical exchange‐mediated processes compared with SS. Magn Reson Med 77:2280–2287, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Optimization of steady‐state pulsed CEST imaging for amide proton transfer at 3T MRI

“…where is the concentration of the solute protons; is an acquired signal in a z ‐spectrum where RF saturation for CEST is applied to the chemical shift of exchangeable solute protons (); is the corresponding z ‐spectrum signal where the concentration of the solute proton is 0; is an image signal when the flip angle of RF CEST is 0; and is the SD of noise in . Equation () quantifies the saturation effect only contributed by CEST process by subtracting DS and MT effects commonly arising from the 2 types of saturated signals . Acquiring the 2 types of signals is explained in detail at Numerical Simulations and MRI Experiments sections of this paper.…”

“…Although a SS‐pCEST scheme was developed for fast 3D scanning, the parameters of RF data have not been considered in optimization research. In a recent work comparing SS‐pCEST with pCEST, the effects of RF data were also not taken into consideration. According to our experiment results, the flip angle of RF data influences not only CEST detectability but also the optimal parameters of RF CEST .…”

pH Imaging Using Chemical Exchange Saturation Transfer (CEST) MRI

“…Pulse shape and length, inter pulse delay and the number of shape pulses are factors to consider for optimizing the CEST contrast with pulsed CEST. [52] Other alternatives for implementing CEST include steady stated based methods where the saturation is built up using a series of short saturation pulses and repetition times on the order of tens of ms for a gradient echo imaging readouts to achieve sufficient saturation transfer, [53,54] frequency labeled exchange transfer (FLEX), [55] saturation which employs a train of excitation pulses to modulate the solute signal instead of saturating it, SAturation with Frequency Alternating RF Irradiation (SAFARI) or Two-Frequency RF Irradiation which employ a phase or frequency modulated saturation pulse train to isolate the CEST signal [56,57] and spin 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 lock based methods which employ a spin lock instead of saturation pulse. [58] For the image acquisition period, Echo Planar Imaging (EPI) acquisition and Fast Spin Echo (FSE) are the most commonly used acquisition methods for CEST imaging.…”

Fast 3D chemical exchange saturation transfer imaging with variably‐accelerated sensitivity encoding (vSENSE)

“…31 The vSENSE method benefits from the multiple frames acquired from the same imaging volume in a CEST experiment, and the underlying fact that the sensitivity maps used for SENSE reconstruction 32 do not change between frames. Other sequence-oriented [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] and reconstruction-oriented [48][49][50][51] schemes for accelerating CEST imaging have been proposed. vSENSE is a reconstruction-oriented method that can be combined with any of the sequence-oriented methods to achieve even faster acquisitions.…”